In the watchmaking business, the escapement is the mechanism placed between the gear train of a watch movement and its regulator, for example the balance of the watch movement, the function of the escapement being to maintain the oscillations of the balance and control the speed of the gear train.
With reference to FIG. 1, an escapement 1 of a watch with a conventional pallet will briefly be described. Escapement 1 comprises an escapement wheel 2, a pallet 4 and a large roller 6 carrying an impulse pin 8 and a small roller 10 provided with a notch 11 carried by the balance staff 12. The pallet comprises two pallet-stones 14 cooperating with wheel 2 and a lever 16 ending in a fork 17 with three teeth cooperating with the large and small rollers 6 and 10 respectively. These pallet-stones 14 are each secured in a suitable housing or recess 18 provided in arms 19 of pallet 4. Fork 17 comprises two outer teeth 20 which cooperate with impulse pin 8 whereas the inner tooth or dart 22 cooperates with small roller 10 and prevents pallet 4 inadvertently overbanking during the additional oscillation of the balance (not shown). The movements of pallet 4 are limited by pins or stop members 24 against which lever 16 abuts. In the rest position, lever 16 abuts against one of the two stop pins 24. At this moment, one tooth 2d of escapement wheel 2 is pressed against one of pallet-stones 14 as is shown in FIG. 1. The inner tooth 22 of fork 17 is released from notch 11 such that the balance is released and covers a distance in the anti-clockwise direction S, the amplitude of which is determined by the accumulated energy. The balance then sets off in the clockwise direction to release pallet-stone 14 from tooth 2d. After the impulse imparted to pallet-stone 14 by tooth 2d, the pallet covers a small idle distance, called the backlash, until the moment when lever 16 abuts against pin 24. The balance then covers its amplitude, and returns until impulse-pin 8 of large roller 6 drives fork 17, which releases the following impulse. This backlash is a way of ensuring that all the teeth of escapement wheel 2 can pass. The length of the backlash is a function of the penetration p of the tooth of escapement wheel 20 onto the rest plane 27 of pallet-stone 14, said penetration having to be both sufficient to prevent the inadvertent release of escapement wheel 2, for example when there is a shock, and sufficiently slight to ensure that the teeth of escapement wheel 2 are released in all operating circumstances in order to obtain an escapement 1 with optimum yield. The means for securing the pallet-stone are critical for the escapement to operate properly and have to allow the possibility of adjusting penetration P.
In order to attain this object, the pallet-stones are currently secured in their housings by means of shellac bond. Shellac bond is a natural organic adhesive which has the advantage of being able to melt at any time at a low temperature and into a thick sticky liquid and thus enabling the position of the pallet-stones to be adjusted easily and precisely.
The use of shellac bond has, however, some drawbacks. In fact, shellac bond is very sensitive to the chemical agents used for washing movements and consequently releases shellac bond particles which stick to various places on the movement. Depending upon the place, these particles can considerably affect the escapement efficiency and consequently disturb the working of the watch.
Moreover, since it is a natural product, its properties are not constant so that the quality of the shellac bond can vary from one delivery to another.
Furthermore, implementation of shellac bond takes a long time and is difficult, requiring great dexterity to apply the optimum quantity of shellac bond in the pallet housing in order to properly secure the pallet-stones on the pallet.
It is thus a main object of the invention to overcome the aforementioned drawbacks of the prior art by providing a device for securing a pallet-stone on a pallet which secures the pallet-stone reliably without using shellac bond or any other adhesive while allowing the penetration of the pallet-stone into the toothing of the escapement wheel to be easily and precisely adjusted.
It is also an object of the invention to provide a device of this type that is easy to implement and secures the pallet-stone precisely and economically.